Optical generation of radio frequency (RF), millimeter-wave (mmW), and Terahertz (THz) frequency signals often involves mixing two optical beams having frequencies separated by a desired frequency. The mixing may be performed with a nonlinear device such as a high-speed photodetector (PD). Although simply mixing output beams of two lasers operating independently on a photodiode produces high frequency signals, the frequency and phase stability are limited by the stability of the individual lasers. If two independent lasers are used, the line-width of the resulting signal may be difficult to reduce below 1 MHz when using DFB lasers. Such spectral purity may be inadequate for many applications because applications often require a narrow line-width and a stable phase of emitted electromagnetic radiation. To reduce line-width, the laser sources may be phase-locked.
Given that the two lasers operate at different wavelengths, simple laser seeding may not produce a desired frequency separation because the laser seeding would normally produce coherent beams of the same wavelength.